1. Field of the Invention
The present invention relates to lasers and particularly to a diode-pumped, fiber laser doped with preselected rare earth activator ions for producing simultaneous output continuous wave (CW) laser emissions at wavelengths determined by the rare earth activator ions.
2. Description of the Prior Art
The term "self-termination" refers to the phenomenon wherein a continuous wave (CW) output, at an appreciable slope efficiency, from a laser source is deemed unlikely when the electron lifetime in the lower energy level (.tau..sub.lower) is longer than that of upper energy level (.tau..sub.upper). A continuous wave (CW) output at an appreciable slope efficiency is deemed feasible when the electron lifetime in the lower energy level (.tau..sub.lower) is shorter than that of the upper energy level (.tau..sub.upper). In other words, if .tau..sub.upper &gt;.tau..sub.lower then CW output is feasible at an appreciable slope efficiency, for example, greater than 10% and if .tau..sub.upper &lt;.tau..sub.lower then CW output is not feasible at an at an appreciable slope efficiency, for example, greater than 10%.
The self-termination phenomenon can be more readily understood if one considers that if .tau..sub.upper &lt;.tau..sub.lower, then, at some future time, as electrons transfer out faster from the upper energy level into the lower energy level than electrons transfer out of the lower energy level, the lower energy level will eventually become filled to a greater extent than the upper energy level. When the lower energy level becomes filled to a greater extent than the upper energy level, corresponding to the CW lasing transition, then it is considered that the given transition from the upper energy level to the lower energy level has self-terminated. Thus, self-termination makes a CW laser output above an appreciable slope efficiency almost impossible to occur.
Such a self-termination phenomenon with respect to the four level system of Er.sup.3+ is discussed by Quimby et al. in their paper titled Continuous-Wave Lasing on a Self-Terminating Transition published in Applied Optics, Volume 28, No. 1 in January, 1989 at pp. 14. However, Quimby et al. state in their conclusion that ". . . cw [continuous wave] lasing is possible in certain cases when the . . . lifetime of the upper level is shorter than that of the lower laser level . . . " Quimby et al. reach their conclusion by showing that the condition for CW lasing is as follows: EQU .tau..sub.upper &gt;.beta..sub.upper.fwdarw.lower .tau..sub.lower
where .tau..sub.upper is the upper laser energy level electron lifetime, .beta..sub.upper.fwdarw.lower is the branching ratio from the upper laser energy level to the lower laser energy level and .tau..sub.lower is the lower laser energy level electron lifetime. Thus, according to Quimby et al., even if .tau..sub.lower is greater than .tau..sub.upper, it is possible to obtain CW lasing for a transition if the condition, .tau..sub.upper &gt;.beta..sub.upper.fwdarw.lower .tau..sub.lower, can be satisfied.
Carter et al. in their paper titled Thulium-Doped Fluorozirconate Fiber Lasers Operating at Around 0.8, 1.47, 1.9, and 2.3 .mu.m Pumped at 0.79 .mu.m published in OSA Proceedings on Advanced Solid-State Lasers, Volume 10 in 1991 at pp. 218-221 explore the .sup.3 H.sub.4 .fwdarw..sup.3 F.sub.4 laser transition corresponding to 1.47 .mu.m in a thulium doped fluorozirconate laser fiber. Carter et al. state on page 220 that
Initial experiments have . . . been carried out on laser oscillation at 1.47 .mu.m on the .sup.3 F.sub.4 -.sup.3 H.sub.4 transition using a cavity consisting of mirrors of &gt;99% reflectivity butted at either end of the fiber. Self-terminating pulsed laser emission is observed at 1.47 .mu.m for a threshold power of 250 mW incident on the launch microscope objective (175 mW being launched into the fiber). The reason for the laser emission being self-terminating is that the lower laser level [.sup.3 F.sub.4 ] has a longer lifetime (6.4 ms) than the upper level [.sup.3 H.sub.4 ] (1.1 ms). However, if the fib[er] was pumped sufficiently hard, CW operation could be enforced. The threshold for CW operation was.apprxeq.480 mW incident pump power. As a result of the low value of output coupling, the efficiency of this device was rather low, with &lt;1 mW of output power being measured.
Carter et al. establishes that the thulium doped fluorozirconate fiber laser is prone to self-termination limiting the slope efficiency of any CW laser output at 1.4-1.5 .mu.m. In fact, only a negligible output at &lt;1 mW is observed for the .sup.3 H.sub.4 .fwdarw..sup.3 F.sub.4 transition corresponding to the output at 1.47 .mu.m. Id at 220.